These turned out to be useful recently

With the -e parameter, you can specify a range of additional values not directly related to rsync but to ssh, for instance in my case where I have multiple ssh keys and some are passwordless, some are not. In this case I wanted to connect via password-less login to another internal server for a midnight rsync backup..

rsync -(other usual parameters) -e "ssh -i /path/to/other/id_rsa file/" /local/backup/dir/ /remote/backup/dir

Using -e you can also specify alternate remote ssh user among other things. check the man page

-ignore-existing is also useful if you've initially grabbed files with scp and don't want rsync to waste time and bandwidth overwriting existing images, only wanting to get new ones... kind of an emergency tool :)